Method for determining the position in a cassette recorder

The invention relates to a method for sensing a position of a mechanism that controls a storage medium such as, for example, magnetic tape recorder preamble of claim 1. The method is especially applicable to magnetic tape devices for analog and/or digital recording of video and/or audio data as well as data recorders which comprise position switches. This is the case, for example, with cassette identification or loading mechanism position acknowledgement (feedback).

With a tape loading mechanism in a magnetic tape recorder having a rotating head drum (cylinder), the tape is drawn out from the cassette by means of the mechanism and wound around the head drum over an angle of approximately 180 degrees. During the eject (unloading) procedure the tape is guided back into the direction of the cassette. Various systems are known for the mechanism such as, for example, the M-loading in the case of VHS and the C-loading for Video 2000. Differing positions are required by both systems for the various operating states such as playback, pause, winding, cassette eject, etc. Each individual position represents a switching status which serves as a feedback for the controlling electronics. Here, indexing (shift) switches are mainly used, or coding switches or separate switches too. The number of feedback lines in conventional devices is about three or four, depending on the complexity of the mechanism.

In practice it has become apparent that the differences between the voltage values assigned to the individual positions are very small, whereby the evaluation becomes unreliable. Greater reliability can be achieved with very accurate resistors and a stabilized voltage at the resistor network and the reference input of the analog-to-digital converter serving for the evaluation. However, the cost of the circuit is, thereby, increased considerably. It is the object of the invention to guarantee a reliable evaluation despite the small differences between the individual determined voltage values and without needing highly accurate resistors and a stabilized voltage at the resistor network and the analog-to-digital converter. Developments of the invention are given in the subclaims.

Therefore, with the method according to the invention, highly accurate resistors or stabilized voltages are replaced by an intentionally introduced calibration procedure. Practically all deviations among the resistor values or the operating voltage are detected and neutralized through this calibration procedure in that the voltage values measured at the respective positions in the calibrating operation are stored and used as desired (target) values during normal operation. Therefore, it is not the theoretically correct desired values which are used for the comparison in the evaluation electronics but, on the contrary, those values are used as desired values which actually occur at the individual positions in practice and which are determined and stored in a calibration procedure. The resistor network can then be realized with conventional, cost-effective resistors having a greater tolerance, while no highly stable voltage is necessary as the operating voltage for the network and the analog-to-digital converter.

FIG. 1 shows a simplified circuit diagram for determining the position of a mechanism of a tape recorder;

FIG. 2 is a second embodiment of a circuit for determining the position of a mechanism, used for carrying out an aspect of the invention, that utilizes position switches of a resistor network such as shown in FIG. 1; and

FIG. 3 shows a numerical example for voltage values produced at different positions of the mechanism by the arrangement of FIG. 2.

In FIG. 1 the slider (wiper) contact `A'` is guided by motor M through the linkage line WV over the position transmitter (indicator) B which has three contact paths 1, 2, 3 as well as a contact path 4 clamped to ground. According to the respective position of the sliding contact `A`, the contact paths 1, 2, 4 are connected to the contact path 4. By wiring each of the contact paths 1, 2, 3 always with a resistor to the supply voltage, switching signals can be picked up at lines a, b, c upon connecting contact path 4 to ground, whereby said signals are then fed to the evaluation electronics AE. Each contact path such as contact path 1, 2 or 3, together with wiper contact A forms a corresponding position switch. The resistor network is formed by the resistors R1, R2, R3, each of equal size--22 kOhm, and is connected to an operating voltage of +5 V. An advantageous simplification is the reduction of the lines to one single line, whereby differing direct voltages are created for various switching states via a resistor network. The evaluation electronics AE contain an analog-to-digital converter which converts the analog voltages supplied by the lines `a`, b, c into digital values which can be differentiated.

Such a resistor network with resistors R1, R2, R3, R4 and R5 is illustrated in FIG. 2, whereby the resistors have the folowing values:

R1=27 kOhm

R2=47 kOhm

R3=100 kOhm

R4=220 kOhm

R5=39 kOhm.

The common output e of the resistor matrix (array) R1 through R5 is connected to the input of the analog-to-digital converter W within the evaluation electronics AE.

FIG. 3 shows the respective ensuing direct voltages for the individual possible switching states in numerical and graphical form. If the voltage values which actually occur are known to the evaluation electronics AE, then the limiting values for the recognition of each individual voltage value can be accurately defined. During a test phase, for example, assembly of the device or servicing, a calibration run (operation) is carried out for the analog-to-digital converter W with the connected network. The values measured thereby are stored and used during each insert/eject (loading/unloading) procedure as desired values.

The calibrating operation begins at a prescribed starting position. This "zero position" is triggered first upon calling up the calibration function, hereinafter designated phase `A`. There are various possibilities for this. For the case presented here, the zero position is the eject position:

1. The eject operation is to be continued until the zero position is reached at all costs. A time criterion is effective in this operation.

2. Carry out the operation until there is a clear increase in the loading motor current. Here, we are dealing with a current inquiry.

3. Continue the operation until a higher order switching position is reached, for example, "cassette ejected" position. Here, we are dealing with a switch inquiry.

The next and subsequent step, namely phase B, is the actual calibrating operation in which the loading mechanism is driven at low speed in cyclic operation, i.e. two cycles "off" and one cycle "on", in the direction of the tape load end position. Any changes in voltage in the measurement line above a threshold value leads to the loading mechanism being stopped, the position counter state being incremented and multiple inquiry of the applied direct voltage value. Following this, an average value is calculated and this value is assigned to a loading position number. These steps are continued until all positions designed for have been measured.

A following step in a further phase C is carried out, but this is not absolutely necessary, however. For this, the loading mechanism is operated at a low speed in the direction of the tape unload end position and, in principle, continued as in the preceeding operation. Merely the position counter state is decremented for each change.

Upon attaining condition 1 (time) or 2 (current) or 3 (switch), the position counter has now reached the starting position again. A difference implies an incorrect measurement and the procedure (phases A, B, C) is carried out once again. The insertion mechanism is now in the starting position again so that a cassette can be inserted immediately in order that further tests can be performed. The values which have been measured and assigned are deposited in a non-volatile memory, for example, an EEPROM, and read out from this after every switch-on procedure. As the calibrating operation is preferably carried out without a tape in order to avoid, for example, superfluous things such as tensioning the tape or switching on the head drum, the warning signals such as "no cassette", "tape end", "tape start", "broken tape" must be deliberately ignored.